This proposal focuses on the application of pericyclic reactions in new methodology for the synthesis of several important classes of organic compounds. The synthesis and reactions of cyclopropylboranes will be investigated, and this chemistry will be applied in general, stereocontrolled routes to a variety of substituted and functionalized cyclopropane derivatives. Highly stereocontrolled [4+1] annulation strategies involving accelerated sigmatropic rearrangements will be developed as general methods for the synthesis of five-membered carbocyclic compounds. Electrocyclic reactions and [2+2] and [4+2] cycloadditions of vinylketenes will be investigated in the context of general methods for the synthesis of alicyclic and aromatic compounds. The application of this methodology in a highly convergent stereocontrolled total synthesis of the antiviral antibiotic ascochlorin will be examined.